Historic Earthquakes

Northwestern Mona Passage
1918 October 11 14:14 UTC
Magnitude 7.5

This was one of the most violent earthquakes felt on Puerto Rico since its occupation by Europeans. Immediately following the shock a tsunami broke upon the shore, drowning many persons and destroying many native dwellings. Property damage was estimated at about $4,000,000 and 116 lives were lost.

Most extensive property damage reports were recieved from Aguada and Anasco, both located on alluvium in Western Puerto Rico. In Aguada, masonary buildings were largly demolished. Some walls, severely cracked by the main shock, were thown down by the aftershock on Oct. 24. The church, built prior to 1876, was entirely destroyed. Some concrete walls and foundations without reinforcement were wrecked. A one-story reinforced concrete school was practically undamaged, and frame buildings were not damaged appreciably.

At Anasco, all brick buildings were destroyed or condemned. Several concrete structures with no reinforcement were generally wrecked during aftershocks, but those which were well reinforced sustained little damage. Wood frame buildings were not damaged except where timbers had rotted.

North of Aguada at Aguadilla, most of the buildings on the alluvium were badly damaged or destroyed. As a rule, masonry structures were badly cracked.

At Mayaguez, south of Anasco, the brick Catholic Church had to be razed, but the brick Presbyterian Church was only slightly damaged. Several persons were killed in the collapse of a two-story, poorly built concrete cigar factory.

Many bridges were damaged; railroad tracks were bent and displaced; pipelines and flumes were wrecked; and tall brick chimmneys were thrown down. Two cable links were broken in Mona Passage. Before the tsunami arrived, the ocean withdrew and exposed reefs and stretches of seafloor never visible during low tide.When the water returned, it reached heights that were equally high above normal, perhaps 9.5 feet.

Abridged from Earthquake History of the United States, Publication 41-1, Revised Edition (Through 1970), Reprinted 1982, With Supplement (1971-80). Edited by Jerry L. Coffman and Carl A. von Hake, NOAA, Environmental Data And Information Service, and Carl W. Stover, U.S. Geological Survey. U.S. Department of Commerce, National Oceanic and Atmospheric Administration and U.S. Department of the Interior, Geological Survey. Boulder, Colorado, 1982.

The epicenter was located northwest of Aguadilla in the Mona Canyon (between Puerto Rico and the Dominican Republic). This earthquake had an approximate magnitude of 7.5 on the Richter scale and was accompanied by a tsunami ("tidal" wave) which got up to 6 meters (19.5 feet) high. Damage was concentrated in the western area of the Island because this was the closest zone to the earthquake. The earthquake killed about 116 people and caused more than 4 million dollars of damage. Numerous houses, factories, public buildings, chimneys, bridges and other structures suffered severe damage.

Translated from "Terremoto," written by Jose Molinelli Freytes, Puerto Rico Civil Defense, under the auspices of the Federal Emergency Management Agency (FEMA).